linux-kernel - Re: [PATCH] mm: avoid bothering interrupted task when charge memcg in softirq

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Date:   Sun, 15 Jul 2018 10:10:10 +0800
From:   Yafang Shao <laoar.shao@...il.com>
To:     Shakeel Butt <shakeelb@...gle.com>
Cc:     Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Cgroups <cgroups@...r.kernel.org>, Linux MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: avoid bothering interrupted task when charge memcg in softirq

On Sat, Jul 14, 2018 at 11:38 PM, Shakeel Butt <shakeelb@...gle.com> wrote:
> On Sat, Jul 14, 2018 at 1:32 AM Yafang Shao <laoar.shao@...il.com> wrote:
>>
>> try_charge maybe executed in packet receive path, which is in interrupt
>> context.
>> In this situation, the 'current' is the interrupted task, which may has
>> no relation to the rx softirq, So it is nonsense to use 'current'.
>>
>
> Have you actually seen this occurring?

Hi Shakeel,

I'm trying to produce this issue, but haven't find it occur yet.

> I am not very familiar with the
> network code but I can think of two ways try_charge() can be called
> from network code. Either through kmem charging or through
> mem_cgroup_charge_skmem() and both locations correctly handle
> interrupt context.
>

Why do you say that mem_cgroup_charge_skmem() correctly hanle
interrupt context ?

Let me show you why mem_cgroup_charge_skmem isn't hanling interrupt
context correctly.

mem_cgroup_charge_skmem() is calling  try_charge() twice.
The first one is with GFP_NOWAIT as the gfp_mask, and the second one
is with  (GFP_NOWAIT |  __GFP_NOFAIL) as the gfp_mask.

If page_counter_try_charge() failes at the first time, -ENOMEM is returned.
Then mem_cgroup_charge_skmem() will call try_charge() once more with
__GFP_NOFAIL set, and this time if If page_counter_try_charge() failes
again the '
force' label in  try_charge() will be executed and 0 is returned.

No matter what, the 'current' will be used and touched, that is
meaning mem_cgroup_charge_skmem() isn't hanling the interrupt context
correctly.

Pls. let me know if I miss something.

Thanks
Yafang